1. Field of the Invention
This invention relates to arrangements for supporting drilling barges and more particularly it concerns a novel method for securely positioning a shallow water drilling barge on an underwater bed of low bearing capacity.
2. Description of the Prior Art
Many oil deposits in the United States and in other countries have been found to be located in areas characterized by shallow water overlying a bed of silty soil having very low bearing capacity, e.g. less than 8,900 pounds per square foot (3,900 kg/m.sup.2). The technique most widely used for drilling in these areas has been to float a drilling barge having shallow draft into position over the drilling site and then to flood the barge hull and allow it to settle onto the underlying soil. Sometimes the barge is "posted", that is, its deck containing the drilling materials and equipment is elevated above the hull so that when the hull is resting on the bottom, the deck and the materials and equipment carried on it will be maintained up above the water level.
Problems have been encountered where the soil under the water has low bearing capacity. Although the soil may be able to support the barge initially, it often is incapable of maintaining support as drilling progresses and the drill string extending down from the barge into the earth extends in length to impose a steadily increasing "hook load" on the barge. This hook load may eventually exceed the supporting capability of the soil, and the barge will no longer be stably supported but instead will begin to shift or sink.
One prior technique which has been used to overcome this problem of low soil bearing capacity involved the building up of a pad of suitable dense material, e.g. oyster shells, on the water bottom before the barge was brought to the site. The pad would provide a large surface area to reduce the unit pressure on the underlying soil. In many instances this technique is prohibitively expensive. Also, depending on the soil conditions, even the pad itself would gradually sink into the underlying soil.
It also has been proposed to provide a jackup barge to support the drilling barge during drilling operations. This technique, as described in Drilling Contractor Volume 37, No. 11, November, 1981 on pages 57 and 60, would involve the floating of a "shallow water jackup leveling barge" to the drilling site, jacking vertical legs down from the barge and into the underlying soil until the barge is lifted up out of the water, pinning the elevated barge to the legs and ballasting the barge with water to drive the legs down into the soil. The barge would then be lowered down along the legs until it is submerged. A drilling barge would then be floated over the submerged jackup leveling barge and would be flooded so that it comes to rest on the jackup leveling barge.
The above described technique would have the disadvantage of requiring substantially all of the weight of both the jackup levelling barge and the drilling barge to be supported on the jackup legs. In many instances the underlying earth will not provide sufficient bearing resistance to support the loads imposed through the legs except at depths which are for beyond their practical length. Also, the raising of the jackup barge and the ballasting to drive the legs down into the underlying soil could create unstable and potentially dangerous conditions.